Last summer, while I was preparing lectures for Intro Bio*, I saw something on twitter about Pablo Escobar’s hippos. The short version of the story, which is summarized in this BBC piece, is that Pablo Escobar, the Colombian drug baron, imported hippos and various other animals to his ranch, Hacienda Napoles. (This was not legal, but clearly Escobar wasn’t overly concerned with legality.) According to the BBC article and the video here, the other animals were all removed and taken to zoos, but at least some of the hippos remained and eventually broke through the ranch’s electric fence. The hippos appear to be doing very well in Colombia, reproducing much sooner and much more often than in their native range.
This seemed like a great unifying theme for a lecture on population ecology. This came up on twitter this morning (after I tweeted this xkcd comic on the population dynamics of fairies). I figured it would help to flesh it out in a blog post, rather than attempting to cover this 140 characters at a time!
I started the population ecology lecture by showing a clip of the video (link above), and then asked the students to consider how the size of the Colombian hippo population would change over time. I felt like this example helped students understand right away why we might want to predict changes in population size. Before we got into predicting population size, though, I used the example to do a clicker question on density vs. abundance: I asked whether hippo density and/or abundance had changed on the day the hippos broke through the Hacienda Napoles fence; students had pre-reading related to density and abundance, and this question let me make sure they had followed this. (Most had, but there was more confusion than I’d have hoped, and this let me address that.) I could have also used this clicker question as a question on the pre-class quiz instead.
I gave the students a very simple scenario where I essentially turned the hippos into annual plants (for simplicity of math), and then asked them to project population growth. I then introduced the concept of lambda and asked them to calculate it for the hippos and then introduced equations for geometric population growth.
I moved away from the hippos for a bit to cover discrete vs. continuous growth, to cover lambda and r more generally, and to briefly teach calculus. (About 1/3 of students in my class have not had calculus.) I ended this section by coming back to the hippos to ask a clicker question asking the students if r for the Colombian hippos was greater than zero, zero, less than zero, or unknown.
We then continued with the hippos to calculate exponential growth of the Columbian hippos, and to have them plot out on their own what the figures for time vs. population size and time vs. # hippos born per year would look like. This fit with one of my key goals for the semester – to get students to be comfortable with figures.
I then did a clicker question that was designed to get the students to realize just how quickly exponential growth can get a population to huge, unrealistic numbers. I asked them to predict the hippo population size after 20 years, with the same semi-made-up demographic parameters I’d given them earlier – it would have millions of hippos, which clearly has not happened. I use that (along with the picture showing the highest hippo density that I could find – a picture like this one would work really well) to ask the students whether Colombia will be overrun by hippos (hint: probably not), and to introduce a really key ecological concept: exponential population growth cannot occur indefinitely because resource limitation will eventually stop exponential population growth.
At the end of the lecture, I covered population management, and left the students with the task of considering how they would control the Colombian hippo population if they were put in charge of it. (The BBC article goes into this and includes the quite serious suggestion that the hippos could be viewed as a food source.)
When I wrote the lecture, I wasn’t planning on extending the hippo theme beyond that one class, but then this excellent feature of work on hippos in Africa came out. So, in the ecosystem ecology unit, we came back to hippos, and I presented some of the information from that piece. A summary of that piece is in this tweet:
I love that the cartoon shows a hippo in the process of releasing nutrients into the water! It’s also hard not to be impressed by the image shown in the Science piece of a Milwaukee pool – pristine one day, completely darkened by hippo dung and urine the next – or by the estimate that hippos release 36 tons of feces a day into the Mara River. This leads to the obvious question of what effects Pablo Escobar’s hippos are having on Colombian rivers, but I don’t know of anyone working on that. (If no one is, someone should!) The students also really enjoyed hearing about why hippos have been understudied in the past – they were particularly amused when I pointed out that, since hippos have no necks, radiocollars don’t really work on them. This fit with another goal of mine for the semester — helping students understand how science is done and how we come to know the information that I present to them in class.
So, overall, I think Pablo Escobar’s hippos made a useful, entertaining example, and I plan on using it again next year. It also helps explain why I was so amused when I pulled out cookie cutters over Thanksgiving break and found a hippo!:
Postscript: (added 29 January 2015) A warning for the parents. If you’ve read this book as many times as I have, you will be very, very tempted to say “All the hippos go berserk!” at some point while talking about exponential growth of hippos.
*I was preparing lectures in the summer, even though I wasn’t giving my first lecture until the end of October, because I knew that the workload involved in flipping the classroom would be really high.